Mutations in iron-sulfur cluster proteins that improve xylose utilization

ABSTRACT

There is provided an engineered host cells comprising (a) one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism; and (b) at least one gene encoding a polypeptide having xylose isomerase activity, and methods of their use thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 15/884,449 filed Jan. 31, 2018, which is a continuation of U.S. Ser. No. 14/821,955 filed Aug. 10, 2015, now U.S. Pat. No. 9,920,312, which claims priority from U.S. provisional application 62/035,748 filed on Aug. 11, 2014, the contents of each of which are incorporated by reference in their entirety.

FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was funded, in part, by the United States government under a grant with the Department of Energy, Office of Energy Efficiency and Renewable Energy, Bioenergy Technologies Office, Award No. DE-FC36-08GO18103 to Mascoma and FWP#CEEB007 to Oak Ridge National Laboratory. This invention was also funded, in part, by the Bioenergy Science Center, Oak Ridge National Laboratory, a U.S. Department of Energy Bioenergy Research Center supported by the Office of Biological and Environmental Research, under contract DE-P502-06ER64304. The government has certain rights in the invention.

REFERENCE TO SEQUENCE LISTING SUBMITTED ELECTRONICALLY VIA EFS-WEB

The contents of the attached sequence listing (named 115235-264-seq_listing.txt; of size 112 kb, created Sep. 17, 2019) are incorporated by reference in their entirety.

FIELD OF THE INVENTION

The field of the invention generally relates to engineered host cells comprising (a) one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism; and (b) at least one gene encoding a polypeptide having xylose isomerase activity; and methods of fermenting cellulosic biomass to produce biofuels, including ethanol.

BACKGROUND OF THE INVENTION

Saccharomyces cerevisiae is the primary biocatalyst used in the commercial production of “first generation” fuel ethanol from sugar based substrates such as corn, sugarcane, and sugarbeet. Second generation ethanol production, also known as cellulosic ethanol production, extends the carbohydrate source to more complex polysaccharides, such as cellulose and hemicellulose, which make up a significant portion of most plant cell walls and therefore most plant material.

Feedstocks commercially considered for second generation ethanol production include wood, agriculture residues such as corn stover and wheat straw, sugarcane bagasse and purpose grown materials such as switchgrass. The cellulose and hemicellose must be hydrolyzed to monomeric sugars before fermentation using either mechanical/chemical means and/or enzymatic hydrolysis. The liberated monomeric sugars include glucose, xylose, galactose, mannose, and arabinose with glucose and xylose constituting more than 75% of the monomeric sugars in most feedstocks. For cellulosic ethanol production to be economically viable and compete with first generation ethanol, the biocatalyst must be able to convert the majority, if not all, of the available sugars into ethanol.

S. cerevisiae is the preferred organism for first generation ethanol production due to its robustness, high yield, and many years of safe use. However, naturally occurring S. cerevisiae is unable to ferment xylose into ethanol. For S. cerevisiae to be a viable biocatalyst for second generation ethanol production, it must be able to ferment xylose.

There are two metabolic pathways of xylose fermentation that have been demonstrated in S. cerevisiae. The pathways differ primarily in the conversion of xylose to xylulose. In the first pathway, the XR-XDH pathway, a xylose reductase (XR) converts xylose to xylitol, which is subsequently converted to xylulose by a xylitol dehydrogenase (XDH). The XR and XDH enzyme pairs tested to date differ in required cofactor, NADH and NADPH, leading to difficulties achieving redox balance. The second commonly tried pathway converts xylose directly to xylulose using a xylose isomerase (XI) with no redox cofactor requirements. XIs from both bacterial and fungal systems have been successfully utilized in S. cerevisiae. Both pathways utilize the same downstream metabolic engineering: up regulation of the native xylulose kinase (XKS1) and four genes of the pentose phosphate pathway, specifically ribulose-phosphate 3-epimerase (RPE1), ribose-5-phosphate ketol-isomerase (RKI1), transaldolase (TAL1), and transketolase (TKL1) (FIG. 1). Use of the XI pathway also commonly entails deletion of the native aldose reductase gene (GRE3) to eliminate product lost to xylitol formation.

Xylose isomerases are known to have several metal ion binding sites, which allows XIs to bind metal ions such as manganese, cobalt, and magnesium. See, e.g., Chang et al., “Crystal Structures of Thermostable Xylose Isomerases from Thermus caldophilus and Thermus thermophilus: Possible Structural Determinants of Thermostability,” J. Mol. Biol 288:623-34 (1999). There is some indication that XIs may also bind iron cations (Fe+), but Fe+ is usually not the preferred or optimal divalent cation. However, intracellular iron regulation and metabolism is known to be a critical function for eukaryotic cells due to iron's role as a redox-active protein cofactor. See, e.g., Outten and Albetel, “Iron sensing and regulation in Saccharomyces cerevisiae: Ironing out the mechanistic details,” Curr. Op. Microbiol. 16:662-68 (2013). Intracellular iron levels are primarily controlled by the iron-sensing transcriptional activators Aft1 and Aft2 in S. cerevisiae. Iron-sulfur (Fe/S) clusters are essential for transcriptional control by Aft1/2 and Yap5 during iron sufficiency. Under sufficient iron levels, Fe/S clusters are synthesized in the mitochondria through the integration of iron, sulfur, and redox control pathways. The Fe/S clusters interact with Grx3, Grx4, Fra1, and Fra2 to inactivate Aft1/2, leading to down regulation of Aft1/2 target genes. Fe/S clusters also are known to activate the expression of Yap5 target genes, including CCC1. Ccc1 stimulates the import of iron and its sequestration in the vacuole.

BRIEF SUMMARY OF THE INVENTION

Aspects of the invention are directed to engineered host cells comprising (a) one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism; and (b) at least one gene encoding a polypeptide having xylose isomerase activity, and methods of their use are described herein.

In some embodiments, the host cell heterologously expresses one or more polypeptides capable of converting xylose to xylulose. In some embodiments, the one or more heterologously expressed polypeptide is a xylose isomerase. In some embodiments, the heterologously expressed polypeptide is a naturally occurring polypeptide. In some embodiments, the heterologously expressed polypeptide is recombinant. In some embodiments, the heterologously expressed polypeptide is a chimeric polypeptide. In some embodiments, the chimeric polypeptide is as described in the related provisional application U.S. 62/035,752 filed on Aug. 11, 2014, which application is hereby incorporated by reference in its entirety.

In some embodiments of the present invention, the heterologously expressed polypeptide has at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and/or 27. In some embodiments, the heterologously expressed polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, or 27. In some embodiments of the present invention, the heterologously expressed polypeptide has at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and/or 41. In some embodiments, the heterologously expressed polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, or 41.

In some embodiments, the heterologously expressed polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and/or 28. In some embodiments, the heterologously expressed polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, or 28. In some embodiments, the heterologously expressed polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and/or 42. In some embodiments, the heterologously expressed polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, or 42. In some embodiments, the polynucleotide sequence is contained in a vector.

In some embodiments, a host cell is engineered to express one or more of the chimeric polypeptides. In some embodiments, the host cell is a yeast cell, e.g. a S. cerevisiae cell. In some embodiments the host cell is further modified to have mutations affecting at least one gene encoding a protein involved in the pentose phosphate pathway. In some embodiments, the host cell has at least one mutation that increases the expression or causes the up-regulation of XKS1, RKI1, RPE1, TKL1, and/or TALL In some embodiments, the host cell has a modification of one or more aldose reductase genes. In some embodiments, the aldose reductase gene is GRE3. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous GRE3 gene. In some embodiments, the aldose reductase gene is YPR1. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous YPR1 gene. In some embodiments, the host cell has a deletion or disruption of all or part of both the endogenous GRE3 gene and the endogenous YPR1 gene. In some embodiments, the host cell has a modification of PGM1 (phosphoglucomutase 1) and/or PGM2. In some embodiments, the host cell overexpresses PGM1 and/or PGM2. In some embodiments, the host cell has increased levels of Pgm1 and/or Pgm2 polypeptide and/or mRNA relative to a comparable host cell lacking a modification of PGM1 and/or PGM2.

In some embodiments, the host cell comprises a deletion or disruption of one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the host cell produces less glycerol than a control recombinant microorganism without deletion or disruption of said one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the one or more endogenous enzymes that function to produce glycerol are encoded by a GPD1 polynucleotide, a GPD2 polynucleotide, or both a GPD1 polynucleotide and a GPD2 polynucleotide. In some embodiments, one or both of the endogenous GPD1 and/or GPD2 genes are modified by mutation or deletion. In some embodiments, the host cell comprises a heterologous ADHE sequence. In some embodiments, the heterologous ADHE is from Bifidobacterium adolescentis. In some embodiments the native STL1 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of STL1. In some embodiments the host cell comprises an ortholog of the native STL1. In some embodiments the native ACS2 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of ACS2. In some embodiments the host cell comprises an ortholog of the native ACS2 or ACS1 gene.

In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism. In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding an iron uptake protein, iron utilization protein, and/or an iron/sulfur (Fe/S) cluster biosynthesis protein. In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding a polypeptide affecting iron metabolism or Fe/S cluster biosynthesis. In some embodiments, the host cell is a recombinant yeast cell. In some embodiments, the recombinant yeast cell comprises one or more mutations in one or more of an endogenous gene selected from the group ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, GREX4, CCC1, and combinations thereof. In some embodiments, the recombinant yeast cell comprises one or more mutations in one or more of an endogenous gene which is homologous to one or more of an S. cerevisiae gene selected from the group ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, GREX4, and CCC1. In some embodiments, the recombinant yeast cell comprises a mutation in the endogenous AFT1 or AFT2 gene that results in iron-independent activation of the iron regulon such as the AFT1-1^(up) or AFT2-1^(up) alleles (Rutherford et al., “Aft1p and Aft2p mediate iron-responsive gene expression in yeast through related promoter elements,” JBC 278(30):27636-43 (2003)). In some embodiments, the recombinant yeast cell comprises a deletion or disruption of YAP5 and/or CCC1. In some embodiments, the recombinant yeast cell comprises a deletion or disruption of YAP5 and/or CCC1 and/or a mutation in the endogenous AFT1 or AFT2 gene that results in iron-independent activation of the iron regulon such as the AFT1-1^(up) or AFT2-1^(up) alleles.

In some embodiments, the host cell comprises one or more mutations in the endogenous ISU1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of D71N, D71G, and S98F, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:29. In some embodiments, the host cell comprises one or more mutations in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:31. In some embodiments, the host cell comprises one or more mutations in the endogenous NFS1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of L115W and E458D, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:33.

In some embodiments, the host cell has a modification of PGM1 (phosphoglucomutase 1) and/or PGM2, as described in the related provisional application filed on Aug. 11, 2014, which application is incorporated by reference in its entirety. In some embodiments, the host cell overexpresses PGM1 and/or PGM2. In some embodiments, the host cell has increased levels of Pgm1 and/or Pgm2 polypeptide and/or mRNA relative to a comparable host cell lacking a modification of PGM1 and/or PGM2.

In some embodiments, the host cell expresses one or more heterologous genes encoding a protein that is associated with iron metabolism. In some embodiments, the heterologous gene confers on the recombinant yeast cell an increased ability to utilize xylose as compared to a similar yeast cell lacking the heterologous gene. In some embodiments, the heterologous gene is AFT1, AFT2, and/or an orthologue thereof. In some embodiments, the heterologous gene encodes a polypeptide having iron transport activity. In some embodiments, the heterologous gene encodes a protein that increases the activity and/or expression of Aft1 and/or Aft2. In some embodiments, the heterologous gene is a target of Aft1 and/or Aft2. In some embodiments, the heterologous gene is constitutively expressed. In some embodiments, the heterologous gene is overexpressed. In some embodiments, the heterologous gene encodes a protein that suppresses a gene or protein that suppresses Aft1 and/or Aft2 activity and/or expression. In some embodiments, the heterologous gene encodes a protein that suppresses a gene or protein that suppresses the activity and/or expression of one or more downstream targets of Aft1 and/or Aft2.

In some embodiments, a yeast strain is used as the host cell. In some embodiments, the background of the yeast strain is an industrial yeast strain. One having ordinary skill in the art would be aware of many potential known yeast strains that can be modified according to the present invention, and this invention contemplates all such potential background yeast strains.

In some embodiments of the invention, the recombinant host cell is used to produce a fermentation product from a cellulosic or lignocellulosic material. In some embodiments, the fermentation product is ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, a cephalosporin, or a combination thereof. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof.

One aspect of the invention is directed to a composition comprising a lignocellulosic material and a recombinant yeast host cell comprising one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism and at least one gene encoding a polypeptide having xylose isomerase activity. Another aspect of the invention is directed to a media supernatant generated by incubating a recombinant yeast host comprising one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism and at least one gene encoding a polypeptide having xylose isomerase activity with a medium containing xylose as the only carbon source. In some embodiments, the medium comprises a cellulosic or lignocellulosic material. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, saw mill or paper mill discards, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

FIG. 1 depicts a schematic representation of xylose fermentation in genetically engineered S. cerevisiae.

FIG. 2 depicts a schematic representation of the role of Fe/S clusters in intracellular iron metabolism. See Outten and Albetel, “Iron sensing and regulation in Saccharomyces cerevisiae: Ironing out the mechanistic details,” Curr. Op. Microbiol. 16:662-68 (2013).

FIGS. 3A-3C provide examples of the relative growth of xylose utilizing yeast strains (XUS) with various mutations in genes encoding proteins associated with intracellular iron metabolism, specifically YFH1 (FIG. 3A), ISU1 (FIG. 3B), and NFS1 (FIG. 3C).

FIGS. 4A-4B provide examples of the relative growth of xylose utilizing yeast strains (XUS) with heterozygous and homozygous mutations in genes encoding proteins associated with intracellular iron metabolism, specifically ISU1 (FIG. 4A) and ISU1 and YFH1 (FIG. 4B), in two XUS strains.

FIG. 5 provides examples of the relative growth of xylose utilizing yeast strains heterologously expressing selected xylose isomerase genes, including those from B. thetaiotaomicron (BtXI), Piromyces (PiXI), C. aberensis (CaXI), P. ruminicola (PrXI), P. distasonis (PdXI), XYM2, A. defectiva (AdXI), Lachnoanaerobaculum saburreum (LsXI), Clostridium phytofermentans (CpXI), and Lactobacillus xylosus (LxXI). The growth levels for of each xylose utilizing yeast strain are show with (hashed bars) and without (solid bars) the T163P mutation of YFH1.

FIGS. 6A-6B provide examples of the relative growth of yeast cells heterologously expressing selected xylose isomerases (chromosomally integrated) including those from CX355=chimeric xylose isomerase 355, CX1224=chimeric xylose isomerase 1224, Ad=Abiotrophia defectiva, Bt=Bacteroides thetaioatomicron, Pe=Piromyces, Ls=Lachnoanaerobaculum saburreum with and without a mutation in YFH1. The growth levels for of each xylose utilizing yeast strain are show with (FIG. 6A) and without (FIG. 6B) the T163P mutation of YFH1.

FIG. 7 provides examples of the relative growth of xylose utilizing yeast strains (XUS) with various mutations in genes encoding proteins associated with intracellular iron metabolism, specifically AFT1, and ccc1.

FIG. 8 provides examples of the relative ethanol production of xylose utilizing yeast strains (XUS) grown in glucose/xylose media with and without iron addition

FIG. 9 provides examples of in vitro xylose isomerase activity assay of xylose utilizing yeast strains (XUS).

DETAILED DESCRIPTION OF THE INVENTION I. Definitions

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art of microbial metabolic engineering. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, exemplary methods, devices and materials are described herein.

The embodiment(s) described, and references in the specification to “one embodiment”, “an embodiment”, “an example embodiment”, etc., indicate that the embodiment(s) described can include a particular feature, structure, or characteristic, but every embodiment does not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is understood that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described.

The description of “a” or “an” item herein refers to a single item or multiple items. It is understood that wherever embodiments are described herein with the language “comprising,” otherwise analogous embodiments described in terms of “consisting of and/or “consisting essentially of are also provided. Thus, for example, reference to “a polynucleotide” includes a plurality of such polynucleotides and reference to “the microorganism” includes reference to one or more microorganisms, and so forth.

A “fragment” refers to any portion of a nucleic or amino acid sequence that is less than the entire sequence. A fragment of a nucleotide or an amino acid sequence can be any length of nucleotides or amino acids that is less than the entire length of the cited sequence and more than two nucleotides or amino acids in length. In some embodiments, the fragment can be from a donor sequence.

A “vector,” e.g., a “plasmid” or “YAC” (yeast artificial chromosome) refers to an extrachromosomal element often carrying one or more genes that are not part of the central metabolism of the cell, and can be in the form of a linear or circular double-stranded DNA molecule. Vectors and plasmids can be autonomously replicating sequences, genome integrating sequences, phage or nucleotide sequences, linear, circular, or supercoiled, of a single- or double-stranded DNA or RNA, derived from any source, in which a number of nucleotide sequences have been joined or recombined into a unique construction which is capable of introducing a promoter fragment and DNA sequence for a selected gene product along with appropriate 3′ untranslated sequence into a cell.

An “expression vector” is a vector that is capable of directing the expression of genes to which it is operably associated.

The term “integrated” as used herein refers to genetic elements that are placed, through molecular biology techniques, into the genome of a host cell. For example, genetic elements can be placed into the chromosomes of the host cell as opposed to in a plasmid carried by the host cell. Methods for integrating genetic elements into the genome of a host cell are well known in the art and include homologous recombination. In some embodiments, more than one copy of the genetic elements are placed into the genome of a host cell. In some embodiments, 2, 3, 4, 5, 6, 7, 8, 9, 10, or more copies of the genetic elements are placed into the genome of a host cell.

The term “heterologous” when used in reference to a polynucleotide, a gene, a polypeptide, or an enzyme refers to a polynucleotide, gene, polypeptide, or an enzyme not normally found in the host organism. “Heterologous” also includes a native coding region, or portion thereof, that is removed from the source organism and subsequently reintroduced into the source organism in a form that is different from the corresponding native gene, e.g., not in its natural location in the organism's genome. The heterologous polynucleotide or gene can be introduced into the host organism by, e.g., gene transfer. A heterologous gene can include a native coding region that is a portion of a chimeric gene including non-native regulatory regions that is reintroduced into the native host. Foreign genes can comprise native genes inserted into a non-native organism, or chimeric genes. A heterologous polynucleotide, gene, polypeptide, or an enzyme can be derived from any source, e.g., eukaryotes, prokaryotes, viruses, or synthetic polynucleotide fragments. The term “heterologous” as used herein also refers to an element of a vector, plasmid or host cell that is derived from a source other than the endogenous source. Thus, for example, a heterologous sequence could be a sequence that is derived from a different gene or plasmid from the same host, from a different strain of host cell, or from an organism of a different taxonomic group (e.g., different kingdom, phylum, class, order, family, genus, or species, or any subgroup within one of these classifications). The term “heterologous” is also used synonymously herein with the term “exogenous.” The term “heterologous expression” refers to the expression of a heterologous polynucleotide or gene by a host.

The term “domain” as used herein refers to a part of a molecule or structure that shares common physical or chemical features, for example hydrophobic, polar, globular, helical domains or properties, e.g., a DNA binding domain or an ATP binding domain. Domains can be identified by their homology to conserved structural or functional motifs. Examples of cellobiohydrolase (CBH) domains include the catalytic domain (CD) and the cellulose binding domain (CBD).

A “nucleic acid,” “polynucleotide,” or “nucleic acid molecule” is a polymeric compound comprised of covalently linked subunits called nucleotides. Nucleic acid includes polyribonucleic acid (RNA) and polydeoxyribonucleic acid (DNA), both of which can be single-stranded or double-stranded. DNA includes cDNA, genomic DNA, synthetic DNA, and semi-synthetic DNA.

An “isolated nucleic acid molecule” or “isolated nucleic acid fragment” refers to the phosphate ester polymeric form of ribonucleosides (adenosine, guanosine, uridine, or cytidine; “RNA molecules”) or deoxyribonucleosides (deoxyadenosine, deoxyguanosine, deoxythymidine, or deoxycytidine; “DNA molecules”), or any phosphoester analogs thereof, such as phosphorothioates and thioesters, in either single stranded form, or a double-stranded helix. Double stranded DNA-DNA, DNA-RNA and RNA-RNA helices are possible. The term nucleic acid molecule, and in particular DNA or RNA molecule, refers only to the primary and secondary structure of the molecule, and does not limit it to any particular tertiary forms. Thus, this term includes double-stranded DNA found, inter alia, in linear or circular DNA molecules (e.g., restriction fragments), plasmids, and chromosomes. In discussing the structure of particular double-stranded DNA molecules, sequences are described herein according to the normal convention of giving only the sequence in the 5′ to 3′ direction along the non-transcribed strand of DNA (i.e., the strand having a sequence homologous to the mRNA).

A “gene” refers to an assembly of nucleotides that encode a polypeptide, and includes cDNA and genomic DNA nucleic acids. “Gene” also refers to a nucleic acid fragment that expresses a specific protein, including intervening sequences (introns) between individual coding segments (exons), as well as regulatory sequences preceding (5′ non-coding sequences) and following (3′ non-coding sequences) the coding sequence. “Native gene” refers to a gene as found in nature with its own regulatory sequences. The terms “gene(s)” or “polynucleotide” or “nucleic acid” or “polynucleotide sequence(s)” are intended to include nucleic acid molecules, e.g., polynucleotides which include an open reading frame encoding a polypeptide, and can further include non-coding regulatory sequences, and introns. In addition, the terms are intended to include one or more genes that map to a functional locus. Also, the terms are intended to include a specific gene for a selected purpose. The gene can be endogenous to the host cell or can be recombinantly introduced into the host cell, e.g., as a plasmid maintained episomally or a plasmid (or fragment thereof) that is stably integrated into the genome. In addition to the plasmid form, a gene can, for example, be in the form of linear DNA or RNA. The term “gene” is also intended to refer to multiple copies of a particular gene, e.g., all of the DNA sequences in a cell encoding a particular gene product.

A nucleic acid molecule is “hybridizable” to another nucleic acid molecule, such as a cDNA, genomic DNA, or RNA, when a single stranded form of the nucleic acid molecule can anneal to the other nucleic acid molecule under the appropriate conditions of temperature and solution ionic strength. Hybridization and washing conditions are well known and exemplified, e.g., in Sambrook, J., Fritsch, E. F. and Maniatis, T. MOLECULAR CLONING: A LABORATORY MANUAL, Second Edition, Cold Spring Harbor Laboratory Press, Cold Spring Harbor (1989), particularly Chapter 11 and Table 11.1 therein (hereinafter “Maniatis”, entirely incorporated herein by reference). The conditions of temperature and ionic strength determine the “stringency” of the hybridization. Stringency conditions can be adjusted to screen for moderately similar fragments, such as homologous sequences from distantly related organisms, to highly similar fragments, such as genes that duplicate functional enzymes from closely related organisms. Post-hybridization washes determine stringency conditions. One set of conditions uses a series of washes starting with 6×SSC, 0.5% SDS at room temperature for 15 min, then repeated with 2×SSC, 0.5% SDS at 45° C. for 30 min, and then repeated twice with 0.2×SSC, 0.5% SDS at 50° C. for 30 min. For more stringent conditions, washes are performed at higher temperatures in which the washes are identical to those above except for the temperature of the final two 30 min washes in 0.2×SSC, 0.5% SDS are increased to 60° C. Another set of highly stringent conditions uses two final washes in 0.1×SSC, 0.1% SDS at 65° C. An additional set of highly stringent conditions are defined by hybridization at 0.1×SSC, 0.1% SDS, 65° C. and washed with 2×SSC, 0.1% SDS followed by 0.1×SSC, 0.1% SDS.

Hybridization requires that the two nucleic acids contain complementary sequences, although depending on the stringency of the hybridization, mismatches between bases are possible. The appropriate stringency for hybridizing nucleic acids depends on the length of the nucleic acids and the degree of complementation, variables well known in the art. The greater the degree of similarity or homology between two nucleotide sequences, the greater the value of Tm for hybrids of nucleic acids having those sequences. The relative stability (corresponding to higher Tm) of nucleic acid hybridizations decreases in the following order: RNA:RNA, DNA:RNA, DNA:DNA. For hybrids of greater than 100 nucleotides in length, equations for calculating Tm have been derived (see, e.g., Maniatis at 9.50-9.51). For hybridizations with shorter nucleic acids, i.e., oligonucleotides, the position of mismatches becomes more important, and the length of the oligonucleotide determines its specificity (see, e.g., Maniatis, at 11.7-11.8). In one embodiment the length for a hybridizable nucleic acid is at least about 10 nucleotides. Preferably a minimum length for a hybridizable nucleic acid is at least about 15 nucleotides; more preferably at least about 20 nucleotides; and most preferably the length is at least 30 nucleotides. Furthermore, the skilled artisan will recognize that the temperature and wash solution salt concentration can be adjusted as necessary according to factors such as length of the probe.

As used herein the term “codon-optimized” means that a nucleic acid coding region has been adapted for expression in the cells of a given organism by replacing one, or more than one, or a significant number, of codons with one or more codons that are more frequently used in the genes of that organism.

The term “percent identity”, as known in the art, is a relationship between two or more polypeptide sequences or two or more polynucleotide sequences, as determined by comparing the sequences. In the art, “identity” also means the degree of sequence relatedness between polypeptide or polynucleotide sequences, as the case can be, as determined by the match between strings of such sequences.

As known in the art, “similarity” between two polypeptides is determined by comparing the amino acid sequence and conserved amino acid substitutes thereto of the polypeptide to the sequence of a second polypeptide. Similarity can be between two full sequences, or between a fragment of one sequence and a fragment of a second sequence wherein the fragments are of comparable length or size, or between a fragment of one sequence and the entirety of a second sequence.

“Identity” and “similarity” can be readily calculated by known methods, including but not limited to those described in: Computational Molecular Biology (Lesk, A. M, ed.) Oxford University Press, N Y (1988); Biocomputing: Informatics and Genome Projects (Smith, D. W., ed.) Academic Press, N Y (1993); Computer Analysis of Sequence Data, Part I (Griffin, A. M., and Griffin, H. G., eds.) Humana Press, N J (1994); Sequence Analysis in Molecular Biology (von Heinje, G., ed.) Academic Press (1987); and Sequence Analysis Primer (Gribskov, M. and Devereux, J., eds.) Stockton Press, NY (1991). Preferred methods to determine identity are designed to give the best match between the sequences tested. Methods to determine identity and similarity are codified in publicly available computer programs. Sequence alignments and percent identity calculations can be performed using the Megalign program of the LASERGENE bioinformatics computing suite (DNASTAR Inc., Madison, Wis.). Multiple alignments of the sequences disclosed herein were performed using the Clustal method of alignment (Higgins and Sharp (1989) CABIOS. 5:151-153) with the default parameters (GAP PENALTY=10, GAP LENGTH PENALTY=10). Default parameters for pairwise alignments using the Clustal method were KTUPLE 1, GAP PENALTY=3, WINDOW=5 and DIAGONALS SAVED=5.

Suitable nucleic acid sequences or fragments thereof (isolated polynucleotides of the present invention) encode polypeptides that are at least about 70% to about 75% identical to the amino acid sequences reported herein, at least about 80%, at least about 85%, at least about 86%, at least about 87%, at least about 88%, at least about 89%, or at least about 90% identical to the amino acid sequences reported herein, at least about 91%, at least about 92%, at least about 93%, at least about 94%, or at least about 95% identical to the amino acid sequences reported herein, or at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% identical to the amino acid sequences reported herein. Suitable nucleic acid fragments are at least about 50%, at least about 55%, at least about 60%, at least about 65%, at least about 70%, at least about 75%, at least about 76%, at least about 77%, at least about 78%, at least about 79%, at least about 80%, at least about 81%, at least about 82%, at least about 83%, at least about 84%, at least about 85%, at least about 86%, at least about 87%, at least about 88%, at least about 89%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at least about 98%, at least about 99%, or about 100% identical to the nucleic acid sequences reported herein. Suitable nucleic acid fragments not only have the above identities/similarities but typically encode a polypeptide having at least 50 amino acids, at least 100 amino acids, at least 150 amino acids, at least 200 amino acids, or at least 250 amino acids.

A DNA or RNA “coding region” is a DNA or RNA molecule which is transcribed and/or translated into a polypeptide in a cell in vitro or in vivo when placed under the control of appropriate regulatory sequences. “Suitable regulatory regions” refer to nucleic acid regions located upstream (5′ non-coding sequences), within, or downstream (3′ non-coding sequences) of a coding region, and which influence the transcription, RNA processing or stability, or translation of the associated coding region. Regulatory regions include promoters, translation leader sequences, RNA processing site, effector binding site and stem-loop structure. The boundaries of the coding region are determined by a start codon at the 5′ (amino) terminus and a translation stop codon at the 3′ (carboxyl) terminus. A coding region can include, but is not limited to, prokaryotic regions, cDNA from mRNA, genomic DNA molecules, synthetic DNA molecules, or RNA molecules. If the coding region is intended for expression in a eukaryotic cell, a polyadenylation signal and transcription termination sequence will usually be located 3′ to the coding region.

An “isoform” is a protein that has the same function as another protein but which is encoded by a different gene and can have small differences in its sequence.

A “paralogue” is a protein encoded by a gene related by duplication within a genome.

An “orthologue” is gene from a different species that has evolved from a common ancestral gene by speciation. Normally, orthologues retain the same function in the course of evolution as the ancestral gene.

“Open reading frame” is abbreviated ORF and means a length of nucleic acid, either DNA, cDNA or RNA, that comprises a translation start signal or initiation codon, such as an ATG or AUG, and a termination codon and can be potentially translated into a polypeptide sequence.

“Promoter” refers to a DNA fragment capable of controlling the expression of a coding sequence or functional RNA. In general, a coding region is located 3′ to a promoter. Promoters can be isolated in their entirety from a native gene, or be composed of different elements derived from different promoters found in nature, or even comprise synthetic DNA segments. It is understood by those skilled in the art that different promoters can direct the expression of a gene in different tissues or cell types, or at different stages of development, or in response to different environmental or physiological conditions. Promoters which cause a gene to be expressed in most cell types at most times are commonly referred to as “constitutive promoters”. It is further recognized that since in most cases the exact boundaries of regulatory sequences have not been completely defined, DNA fragments of different lengths can have identical promoter activity. A promoter is generally bounded at its 3′ terminus by the transcription initiation site and extends upstream (5′ direction) to include the minimum number of bases or elements necessary to initiate transcription at levels detectable above background. Within the promoter will be found a transcription initiation site (conveniently defined for example, by mapping with nuclease SI), as well as protein binding domains (consensus sequences) responsible for the binding of RNA polymerase. Several promoters are specifically identified by the present invention, however, one having ordinary skill in the art would understand that any number of additional promoters capable of driving the expression in yeast would be included in the present invention.

The term “linker” as used herein refers to a series of nucleotides or amino acids that connect one section of the chimeric polynucleotide or polypeptide to another section of the chimeric polynucleotide of polypeptide. In some embodiments, the linker serves a structural function.

A coding region is “under the control” of transcriptional and translational control elements in a cell when RNA polymerase transcribes the coding region into mRNA, which is then trans-RNA spliced (if the coding region contains introns) and translated into the protein encoded by the coding region.

“Transcriptional and translational control regions” are DNA regulatory regions, such as promoters, enhancers, terminators, and the like, that provide for the expression of a coding region in a host cell. In eukaryotic cells, polyadenylation signals are control regions.

As used herein the term “N-terminal region” refers to the portion of the amino acid sequence consisting of the most N-terminal amino acid residue up to the amino acid residue at position n/2, wherein n is the total number of residues in the sequence. As used herein the term “C-terminal region” refers to the portion of the amino acid sequence consisting of the most C-terminal amino acid residue up to the amino acid residue at position n/2, wherein n is the total number of residues in the sequence.

The term “operably associated” refers to the association of nucleic acid sequences on a single nucleic acid fragment so that the function of one is affected by the other. For example, a promoter is operably associated with a coding region when it is capable of affecting the expression of that coding region (i.e., that the coding region is under the transcriptional control of the promoter). Coding regions can be operably associated to regulatory regions in sense or antisense orientation.

The term “expression,” as used herein, refers to the transcription and stable accumulation of sense (mRNA) or antisense RNA derived from the nucleic acid fragment of the invention. Expression can also refer to translation of mRNA into a polypeptide.

The term “lignocellulose” refers to material that is comprised of lignin and cellulose. Examples of lignocelluloses are provided herein and are known in the art. Examples of lignocellulosic materials include but are not limited to corn stover, straw, bagasse, switchgrass, paper, and wood.

The “pentose phosphate pathway” or “PPP” refers to a biochemical pathway that creates NADPH from glucose-6-P. The PPP has both an oxidative phase and a non-oxidative phase. There are several enzymes that have been identified to play a role in the PPP, including but not limited to glucose-6-P dehydrogenase, gluconolactonase, 6-phosphogluconate dehydrogenase, ribulose-5-phosphate isomerase, ribose-5-phosphate ketol-isomerase (RKI1), ribulose-5-phosphate 3-epimerase (RPE1), transketolase (TKL1), and transaldolase (TAL1).

As used herein “xylose isomerase activity” refers to the ability of an enzyme to directly convert xylose to xylulose. A “xylose isomerase” or “XI” as used herein refers to a protein having xylose isomerase activity (EC 5.3.1.5).

The term “chimeric” or “chimera” refers to a polynucleotide or polypeptide having a nucleotide or polypeptide sequence derived from two or more distinct parent sequences. A “parent sequence” or “donor sequence” is a nucleotide or amino acid sequence used as a source sequence to create the chimeric polynucleotide or polypeptide.

As used herein the term “XYM1” or “XYM2” refers to a xylose isomerase coding sequence or polypeptide isolated from an uncultured bacterium as described by Parachin and Gorwa-Grauslund, “Isolation of xylose isomerase by sequence- and function-based screening from a soil metagenome library,” Biotechnology Biofuels 4(49 (2011).

As used herein, the term “anaerobic” refers to an organism, biochemical reaction, or process that is active or occurs under conditions of an absence of gaseous 0₂.

“Anaerobic conditions” are defined as conditions under which the oxygen concentration in the fermentation medium is too low for the microorganism to use it as a terminal electron acceptor. Anaerobic conditions can be achieved by sparging a fermentation medium with an inert gas such as nitrogen until oxygen is no longer available to the microorganism as a terminal electron acceptor. Alternatively, anaerobic conditions can be achieved by the microorganism consuming the available oxygen of fermentation until oxygen is unavailable to the microorganism as a terminal electron acceptor.

“Aerobic metabolism” refers to a biochemical process in which oxygen is used as a terminal electron acceptor to convert energy, typically in the form of ATP, from carbohydrates. Aerobic metabolism typically occurs, for example, via the electron transport chain in mitochondria in eukaryotes, wherein a single glucose molecule is metabolized completely into carbon dioxide in the presence of oxygen.

In contrast, “anaerobic metabolism” refers to a biochemical process in which oxygen is not the final acceptor of electrons generated. Anaerobic metabolism can be divided into anaerobic respiration, in which compounds other than oxygen serve as the terminal electron acceptor, and substrate level phosphorylation, in which no exogenous electron acceptor is used and products of an intermediate oxidation state are generated via a “fermentative pathway.”

In “fermentative pathways”, the amount of NAD(P)H generated by glycolysis is balanced by the consumption of the same amount of NAD(P)H in subsequent steps. For example, in one of the fermentative pathways of certain yeast strains, NAD(P)H generated through glycolysis donates its electrons to acetaldehyde, yielding ethanol. Fermentative pathways are usually active under anaerobic conditions but can also occur under aerobic conditions, under conditions where NADH is not fully oxidized via the respiratory chain.

As used herein, the term “end-product” refers to a chemical compound that is not or cannot be used by a cell, and so is excreted or allowed to diffuse into the extracellular environment. Common examples of end-products from anaerobic fermentation include, but are not limited to, ethanol, acetic acid, formic acid, lactic acid, hydrogen, and carbon dioxide.

As used herein, “cofactors” are compounds involved in biochemical reactions that are recycled within the cells and remain at approximately steady state levels. Common examples of cofactors involved in anaerobic fermentation include, but are not limited to, NAD⁺ and NADP⁺. In metabolism, a cofactor can act in oxidation-reduction reactions to accept or donate electrons. When organic compounds are broken down by oxidation in metabolism, their energy can be transferred to NAD⁺ by its reduction to NADH, to NADP⁺ by its reduction to NADPH, or to another cofactor, FAD⁺, by its reduction to FADH₂. The reduced cofactors can then be used as a substrate for a reductase.

As used herein, a “pathway” is a group of biochemical reactions that together can convert one compound into another compound in a step-wise process. A product of the first step in a pathway can be a substrate for the second step, and a product of the second step can be a substrate for the third, and so on. Pathways of the present invention include, but are not limited to, the pentose phosphate pathway, the xylose utilization pathway, the ethanol production pathway, and the glycerol production pathway. The term “recombination” or “recombinant” refers to the physical exchange of DNA between two identical (homologous), or nearly identical, DNA molecules. Recombination can be used for targeted gene deletion or to modify the sequence of a gene. The terms “recombinant microorganism” and “recombinant host cell” are used interchangeably herein and refer to microorganisms that have been genetically modified to express or over-express endogenous polynucleotides, or to express heterologous polynucleotides, such as those included in a vector, or which have a modification in expression of an endogenous gene.

By “expression modification” it is meant that the expression of the gene, or level of a RNA molecule or equivalent RNA molecules encoding one or more polypeptides or polypeptide subunits, or activity of one or more polypeptides or polypeptide subunits is up regulated or down-regulated, such that expression, level, or activity, is greater than or less than that observed in the absence of the modification.

The term “iron metabolism” refers to the process by which a cell regulates the intracellular level of iron. The term “protein associated with iron metabolism” refers to a protein involved in the regulation of intracellular iron, including, e.g., a protein that imports, exports, binds, and/or sequesters iron or a protein that controls the expression of a gene that encodes for a protein that imports, exports, binds, and/or sequesters iron. The term “Fe/S cluster biosynthesis” refers to the biosynthesis of Fe/S clusters, including, e.g., the assembly and loading of Fe/S clusters. The term “Fe/S cluster biosynthesis genes”, “Fe/S cluster biosynthesis proteins” or “Fe/S cluster biosynthesis pathway” refers to those polynucleotides and/or genes that are involved in the biosynthesis of Fe/S clusters, including, e.g., the assembly and loading of Fe/S clusters.

In one aspect of the invention, genes or particular polynucleotide sequences are partially, substantially, or completely deleted, silenced, inactivated, or down-regulated in order to inactivate the enzymatic activity they encode. Complete deletions provide maximum stability because there is no opportunity for a reverse mutation to restore function. Alternatively, genes can be partially, substantially, or completely deleted, silenced, inactivated, or down-regulated by insertion, deletion, removal, or substitution of nucleic acid sequences that disrupt the function and/or expression of the gene.

II. Xylose Isomerase Polypeptides

The present invention provides host cells comprising (a) one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism and (b) at least one gene encoding a polypeptide having xylose isomerase activity the use thereof. In some embodiments, the host cell heterologously expresses the polypeptide. In some embodiments, the heterologously expressed polypeptide is a naturally occurring polypeptide. In some embodiments, the heterologously expressed polypeptide is recombinant. In some embodiments, the heterologously expressed polypeptide is a chimeric polypeptide.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and/or 27. In some embodiments, the polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, or 27. In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and/or 28. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, or 28. In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and/or 41. In some embodiments, the polypeptide has an amino acid sequence of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, or 41. In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with any one of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and/or 42. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, or 42.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 1. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 1.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 3. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 3.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 5. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 5.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 7. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 7.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 9. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 9.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 11. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 11.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 13. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 13.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 15. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 15.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 17. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 17.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 19. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 19.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 21. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 21.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 23. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 23.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 25. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 25.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 27. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 27.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 35. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 35.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 37. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 37.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 39. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 39.

In some embodiments, the polypeptide has an amino acid sequence having at least 80%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the amino acid sequence of SEQ ID NO: 41. In some embodiments, the polypeptide has an amino acid sequence having 100% sequence identity with the amino acid sequence of SEQ ID NO: 41.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 2. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 2.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 4.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 6. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 6.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 8. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 8.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 10. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 10.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 12. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 12.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 14. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 14.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 16. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 16.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 18. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 18.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 20. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 20.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 22. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 22.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 24. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 24.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 26. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 26.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 28. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 28.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 36. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 36.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 38. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 38.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 40. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 40.

In some embodiments, the polypeptide is encoded by a polynucleotide sequence having at least 50%, at least 55%, at least 60%, at least 65%, at least 70%, at least 75%, at least 76%, at least 77%, at least 78%, at least 79%, at least 80%, at least 81%, at least 82%, at least 83%, at least 84%, at least 85%, at least 86%, at least 87%, at least 88%, at least 89%, at least 90%, at least 91%, at least 92%, at least 93%, at least 94%, at least 95%, at least 96%, at least 97%, at least 98%, or at least 99% sequence identity with the nucleotide sequence of SEQ ID NO: 42. In some embodiments, the polypeptide is encoded by a polynucleotide sequence of SEQ ID NO: 42.

The present invention involves the heterologous expression of one or more polypeptides having xylose isomerase activity. It is understood by one of ordinary skill in the art that any polypeptide having xylose isomerase activity or any polynucleotide encoding such a polypeptide may be used according to the present invention. Accordingly, this invention is not limited to the list of example xylose isomerase polypeptides provided. It is understood that nucleotide sequences encoding any of the polypeptides defined above are expressly included in the present invention. Further, any nucleotide sequence that comprises one or more amino acid substitutions, insertions and/or deletions, but that are within the ranges of identity or similarity as defined herein are expressly included in the invention. However, the polypeptides having xylose isomerase activity share certain conserved motifs. In one embodiment, the nucleotide sequence of the invention encodes a xylose isomerase amino acid sequence comprising a xylose isomerase signature sequence as defined, e.g., by Meaden et al. (1994, Gene, 141: 97-101): VXW[GP]GREG[YSTA] (present at positions 188-196, relative to SEQ ID NO: 11) and [LIVM]EPKPX[EQ]P (present at positions 233-240, relative to SEQ ID NO: 11), wherein “X” can be any amino acid and wherein amino acids in brackets indicates that one of the bracketed amino acids can be present at that position in the signature sequence. A xylose isomerase amino acid sequence of the invention can further comprise the conserved amino acid residues His-103, Asp-106, and Asp-341, which constitute a triad directly involved in catalysis, Lys-236 plays a structural as well as a functional catalytic role, and Glu-234 (relative to SEQ ID NO: 11), which is involved in magnesium binding (Vangrysperre et al., “Localization of the essential histidine and carboxylate group in D-xylose isomerases,” Biochem. J. 265: 699-705(1990); Henrick et al., “Structures of D-xylose isomerase from Arthrobacter strain B3728 containing the inhibitors xylitol and D-sorbitol at 2.5 A and 2.3 A resolution, respectively,” J. Mol. Biol. 208: 129-157 (1989); Bhosale et al., “Molecular and industrial aspects of glucose isomerase,” Microbiol. Rev. 60: 280-300 (1996)). Amino acid positions of the above signature sequences and conserved residues refer to positions in the reference amino acid sequence of the B. thetaiotaomicron xylose isomerase of SEQ ID NO: 11. In amino acid sequences of the invention other than SEQ ID NO: 11, the amino acid positions of one or more of the above signature sequences and conserved residues are present in amino acid positions corresponding to the positions of the signature sequences and conserved residues in SEQ ID NO: 11, for example in a ClustalW (1.83 or 1.81) sequence alignment using default settings. The skilled person will know how to identify corresponding amino acid positions in xylose isomerase amino acid sequences other than SEQ ID NO: 11 using amino acid sequence alignment algorithms as defined hereinabove. These regions and positions will tolerate no or only conservative amino acid substitutions. One having ordinary skill in the art would understand that even conserved motifs can remain functional with conservative amino acid substitutions, and such substitutions are envisioned by the present invention. Amino acid substitutions outside of these regions and positions are unlikely to greatly affect xylose isomerase activity.

Additional structural features common to XIs have been described, e.g., by Chang et al., “Crystal Structures of Thermostable Xylose Isomerases from Thermus caldophilus and Thermus thermophiles: Possible Structural Determinants of Thermostability,” J. Mol. Biol. 288:623-34 (1999), which is incorporated by reference in its entirety, and RCSB Protein Data Bank, “Xylose Isomerase From Thermotoga neapolitana,” http://www.rcsb.org/pdb/explore/explore.do?structureId=1A0E, last accessed Jun. 29, 2014, at 5:15 pm. There are several known metal binding sits in the XI sequence, including at residues Glu-234, Glu-270, His-273, Asp-298, Asp-309, Asp-311, and Asp-341. One having ordinary skill in the art would understand that any deletions or non-conservative substitutions at any one or more of these residues may lead to a decreased functionability of the resulting XI.

In some embodiments, a host cell is engineered to express one or more of the xylose isomerase polypeptides. In some embodiments, the host cell is a fungal cell, e.g. a yeast cell, e.g. a S. cerevisiae cell. In some embodiments the host cell is modified to have mutations affecting at least one gene encoding a protein of the pentose phosphate pathway. In some embodiments, the host cell has at least one mutation affecting the expression of at least one of XKS1, RKI1, RPE1, TKL1, TAL1, or a combination thereof. In some embodiments, the host cell has one or more mutations that correlate with an increase in the expression or an up-regulation of one or more of XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments the host cell can be modified through the heterologous expression of one or more polynucleotides encoding XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments, the host cell has one or more mutations that correlate with a decrease in the expression or down-regulation of one or more of XKS1, RKI1, RPE1, TKL1, and/or TAL1. In some embodiments, the host cell has a modification of an endogenous aldose reductase. In some embodiments, the aldose reductase is GRE3. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous GRE3 gene. In some embodiments, the aldose reductase gene is YPR1. In some embodiments, the host cell has a deletion or disruption of all or part of the endogenous YPR1 gene. In some embodiments, the host cell has a deletion or disruption of all or part of both the endogenous GRE3 gene and the endogenous YPR1 gene. In some embodiments, the host cell has a modification of PGM1 and/or PGM2. In some embodiments, the host cell overexpresses PGM1 and/or PGM2. In some embodiments, the host cell has increased levels of Pgm1 and/or Pgm2 polypeptide and/or mRNA relative to a comparable host cell lacking a modification of PGM1 and/or PGM2. In some embodiments, the host cell is a modified industrial yeast strain.

In some embodiments, the host cell comprises a deletion or disruption of one or more native enzymes that function to produce glycerol and/or regulate glycerol synthesis as described in, e.g., U.S. Patent Application Publication No. 2014/0186930, which is incorporated by reference herein in its entirety. In some embodiments, the host cell produces less glycerol than a control recombinant microorganism without deletion or disruption of said one or more endogenous enzymes that function to produce glycerol and/or regulate glycerol synthesis. In some embodiments, the one or more endogenous enzymes that function to produce glycerol are encoded by a GPD1 polynucleotide, a GPD2 polynucleotide, or both a GPD1 polynucleotide and a GPD2 polynucleotide. In some embodiments, one or both of the endogenous GPD1 and/or GPD2 genes are modified by mutation or deletion. In some embodiments, the host cell comprises a heterologous ADHE sequence. In some embodiments, the heterologous ADHE is from Bifidobacterium adolescentis. In some embodiments the native STL1 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of STL1. In some embodiments the host cell comprises an ortholog of the native STL1. In some embodiments the native ACS2 gene is upregulated by either modifying the promoter of the native copies or by introducing additional copies of ACS2. In some embodiments the host cell comprises an ortholog of the native ACS2 or ACS1 gene.

In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism. In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding an iron uptake protein, iron utilization protein, and/or an iron/sulfur (Fe/S) cluster biosynthesis protein. In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes encoding a polypeptide affecting iron metabolism or Fe/S cluster biosynthesis. In some embodiments, the host cell is a recombinant yeast cell. In some embodiments, the recombinant yeast cell comprises one or more mutations in one or more of an endogenous gene selected from the group ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, GREX4, CCC1, and combinations thereof. In some embodiments, the recombinant yeast cell comprises one or more mutations in one or more of an endogenous gene which is homologous to one or more of an S. cerevisiae gene selected from the group ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, and GREX4. and CCC1. In some embodiments, the recombinant yeast cell comprises a mutation in the endogenous AFT1 gene that results in iron-independent activation of the iron regulon such as the AFT1-1^(up) or AFT2-1^(up) alleles (Rutherford et al., 2003). In some embodiments, the recombinant yeast cell comprises a deletion or disruption of YAP5 and/or CCC1 and/or a mutation in the endogenous AFT1 or AFT2 gene that results in iron-independent activation of the iron regulon such as the AFT1-1^(up) or AFT2-1^(up) alleles. In some embodiments, the host cell comprises one or more mutations in one or more endogenous genes selected from FRA1, FRA2, GREX3, and GREX4, wherein the one or more mutations results in increased Aft1 and/or Aft2 activity. In some embodiments, the increased Aft1 and/or Aft2 activity results in the increased expression of Aft1 and/or Aft2 target genes. In some embodiments, the one or more mutations in AFT1, AFT2, FRA1, FRA2, GREX3, and/or GREX4 prevent or limit AFT1 and/or AFT2 from forming a complex with Grx3, Grx4, Fra1, and/or Fra2.

In some embodiments, the host cell expresses one or more heterologous genes encoding a protein that is associated with iron metabolism. In some embodiments, the heterologous gene confers on the recombinant yeast cell an increased ability to utilize xylose as compared to a similar yeast cell lacking the heterologous gene. In some embodiments, the heterologous gene is AFT1, AFT2, and/or an orthologue thereof. In some embodiments, the heterologous gene encodes a polypeptide having iron transport activity. In some embodiments, the heterologous gene encodes a protein that increases the activity and/or expression of Aft1 and/or Aft2. In some embodiments, the heterologous gene is a target of Aft1 and/or Aft2. In some embodiments, the heterologous gene is constitutively expressed. In some embodiments, the heterologous gene is overexpressed. In some embodiments, the heterologous gene encodes a protein that suppresses a gene or protein that suppresses Aft1 and/or Aft2 activity and/or expression. In some embodiments, the heterologous gene encodes a protein that suppresses a gene or protein that suppresses the activity and/or expression of one or more downstream targets of Aft1 and/or Aft2.

In some embodiments, the host cell comprises one or more mutations in the endogenous ISU1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of D71N, D71G, and S98F, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:29. In some embodiments, the host cell comprises one or more mutations in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:31. In some embodiments, the host cell comprises one or more mutations in the endogenous NFS1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of L115W and E458D, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:33. In some embodiments, the host cell comprises a mutation in the endogenous ISU1 gene that results in a polypeptide comprising the amino acid substitution D71N, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:29; and a mutation in the endogenous YFH1 gene that results in a polypeptide comprising the amino acid substitution T163P, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:31. In some embodiments, the mutation is homozygous. In some embodiments, the mutation is heterozygous.

In some embodiments, the host cell comprises (a) one or more mutations in one or more endogenous genes encoding a protein associated with iron metabolism, iron uptake, iron utilization, and/or an iron/sulfur (Fe/S) cluster biosynthesis; and (b) at least one heterologous gene encoding a polypeptide having xylose isomerase activity. In some embodiments, at least one heterologous polypeptide having xylose isomerase activity is a xylose isomerase. One having skill in the art would understand that any number of known xylose isomerase sequences could be expressed in the host cell of the present invention. In some embodiments the xylose isomerase is a naturally occurring xylose isomerase. In some embodiments, the xylose isomerase is a recombinant polypeptide. In some embodiments, the xylose isomerase is a chimeric polypeptide. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 80% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 83% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 85% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 87% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 90% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 91% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 92% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 93% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 94% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 95% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 96% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 97% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 98% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 99% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 100% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, and 28.

In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 80% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 83% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 85% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 87% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 90% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 91% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 92% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 93% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 94% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 95% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 96% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 97% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 98% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 99% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42. In some embodiments, the xylose isomerase is encoded by a nucleotide sequence that has at least 100% sequence identity with a nucleotide sequence selected from SEQ ID NOs: 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 36, 38, 40, and 42.

In some embodiments, the xylose isomerase has an amino acid sequence that has at least 80% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 83% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 85% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 87% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 90% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 91% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 92% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 93% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 94% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 95% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 96% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 97% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 98% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 99% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 10% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

In some embodiments, the xylose isomerase has an amino acid sequence that has at least 80% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 83% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 85% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 87% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 90% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 91% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41 43. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 92% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 93% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 94% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 95% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 96% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 97% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 98% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41 43. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 99% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41. In some embodiments, the xylose isomerase has an amino acid sequence that has at least 10% sequence identity with an amino acid sequence selected from SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

In some embodiments, the host cell comprises (a) one or mutation in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution; and (b) at least one heterologous gene encoding a polypeptide having xylose isomerase activity, wherein the polypeptide has an amino acid sequence at least about 80%, at least about 83%, at least about 85%, at least about 87%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at about least 98%, at about least 99%, or about 100% identical to the amino acid sequence of SEQ ID NO:1. In some embodiments, the host cell comprises (a) a deletion or disruption of GRE3 and/or YPR1; (b) one or more mutations that correlate with an increase in the expression or up-regulation of one or more of XKS1, RKI1, RPE1, TKL1, TAL1, PGM1 and/or PGM2; (c) one or mutation in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution; and (d) at least one heterologous gene encoding a polypeptide having xylose isomerase activity, wherein the polypeptide has an amino acid sequence at least about 80%, at least about 83%, at least about 85%, at least about 87%, at least about 90%, at least about 91%, at least about 92%, at least about 93%, at least about 94%, at least about 95%, at least about 96%, at least about 97%, at about least 98%, at about least 99%, or about 100% identical to the amino acid sequence of SEQ ID NO:1. In some embodiments, the host cell can be cultured in a medium supplemented with iron. In some embodiments, the host cell can be cultured under conditions that facilitate and/or stimulate the uptake of iron by the host cell. In some embodiments, the host cell can be cultured under conditions that hinder, prevent, block, and/or decrease the export of iron from the host cell.

In some embodiments, the host cell comprises more than one copy of the polynucleotide encoding the polypeptide having xylose isomerase activity. In some embodiments, the host cell comprises two copies, three copies, four copies, five copies, six copies, seven copies, eight copies, nine copies, ten copies, eleven copies, at least twelve copies, at least fifteen copies, or at least twenty copies of the polynucleotide encoding the polypeptide having xylose isomerase activity.

In some embodiments, the polynucleotide can be present in a vector. In some embodiments, the host cell can comprise the polynucleotide within a vector. In some embodiments, the vector is a plasmid. In some embodiments, the host cell can express the polynucleotide from the vector. In some embodiments, the polynucleotide can be incorporated into the genome of the host cell. In some embodiments, the host cell is a fungal cell. In some embodiments, the host cell is a yeast cell. In some embodiments, the host cell is a S. cerevisiae cell.

Certain embodiments of the present invention describe methods for producing a fermentation product. In certain embodiments, the recombinant host cell comprising the polynucleotide or the polypeptide and a mutation in one or more genes encoding a protein associated with iron metabolism is contacted with a carbon source. In some embodiments, the host cell comprises a mutation in one or more genes encoding a protein associated with iron metabolism, and the host cell is contacted with a carbon source and an exogenous source of a polypeptide having xylose isomerase activity. In certain embodiments, the carbon source comprises xylose. In certain embodiments, xylose is the sole source of carbon in the carbon source. In certain embodiments, a fermentation product is produced by contacting the host cell with the carbon source. In certain embodiments, the fermentation product is recovered. In certain embodiments, the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, cephalosporin, or a combination thereof. In certain embodiments, the fermentation product is ethanol.

IV. Codon-Optimization

In some embodiments, the nucleotide sequence of the one or more polynucleotides disclosed in the present invention are codon-optimized for expression in a fungal host cell. In some embodiments, the nucleotide sequence of the polynucleotide is codon-optimized for expression in a yeast host cell. In some embodiments the nucleotide sequence of the polynucleotide is codon-optimized for expression in S. cerevisiae. Codon-optimized polynucleotides can have a codon adaptation index (CAI) of about 0.8 to 1.0, about 0.9 to 1.0, or about 0.95 to 1.0.

In general, highly expressed genes in an organism are biased towards codons that are recognized by the most abundant tRNA species in that organism. One measure of this bias is the “codon adaptation index” or “CAI,” which measures the extent to which the codons used to encode each amino acid in a particular gene are those which occur most frequently in a reference set of highly expressed genes from an organism. The Codon Adaptation Index is described in more detail in Sharp and Li, Nucleic Acids Research 15:1281-1295 (1987), which is incorporated by reference herein in its entirety.

The CAI of codon-optimized sequences used in the present invention corresponds to from about 0.6 to about 1.0, from about 0.7 to about 1.0, from about 0.8 to about 1.0, from about 0.9 to about 1.0, from about 9.5 to about 1.0, or about 1.0. A codon-optimized sequence can be further modified for expression in a particular organism, depending on that organism's biological constraints. For example, large runs of “As” or “Ts” (e.g., runs greater than 4, 5, 6, 7, 8, 9, or 10 consecutive bases) can be removed from the sequences if these are known to effect transcription negatively. Furthermore, specific restriction enzyme sites can be removed for molecular cloning purposes. Examples of such restriction enzyme sites include Pad, Ascl, BamHI, BgIII, EcoRJ and Xhol. Additionally, the DNA sequence can be checked for direct repeats, inverted repeats and mirror repeats with lengths of ten bases or longer, which can be modified manually by replacing codons with “second best” codons, i.e., codons that occur at the second highest frequency within the particular organism for which the sequence is being optimized.

Deviations in the nucleotide sequence that comprise the codons encoding the amino acids of any polypeptide chain allow for variations in the sequence coding for the gene. Since each codon consists of three nucleotides, and the nucleotides comprising DNA are restricted to four specific bases, there are 64 possible combinations of nucleotides, 61 of which encode amino acids (the remaining three codons encode signals ending translation). The “genetic code” which shows which codons encode which amino acids is well known to one of skill in the art. As a result, many amino acids are designated by more than one codon. For example, the amino acids alanine and proline are coded for by four triplets, serine and arginine by six, whereas tryptophan and methionine are coded by just one triplet. This degeneracy allows for DNA base composition to vary over a wide range without altering the amino acid sequence of the proteins encoded by the DNA.

Many organisms display a bias for use of particular codons to code for insertion of a particular amino acid in a growing peptide chain. Codon preference or codon bias, differences in codon usage between organisms, is afforded by degeneracy of the genetic code, and is well documented among many organisms. Codon bias often correlates with the efficiency of translation of messenger RNA (mRNA), which is in turn believed to be dependent on, inter alia, the properties of the codons being translated and the availability of particular transfer RNA (tRNA) molecules. The predominance of selected tRNAs in a cell is generally a reflection of the codons used most frequently in peptide synthesis. Accordingly, genes can be tailored for optimal gene expression in a given organism based on codon optimization.

Given the large number of gene sequences available for a wide variety of animal, plant and microbial species, it is possible to calculate the relative frequencies of codon usage. Codon usage tables and codon-optimizing programs are readily available, for example, at http://www kazusa.or.jp/codon/ (visited Jul. 15, 2014), and these tables can be adapted in a number of ways. See, e.g., Nakamura, Y., et al. “Codon usage tabulated from the international DNA sequence databases: status for the year 2000,” Nucl. Acids Res. 28:292 (2000).

By utilizing one or more available tables, one of ordinary skill in the art can apply the frequencies to any given polypeptide sequence, and produce a nucleic acid fragment of a codon-optimized coding region which encodes the polypeptide, but which uses codons optimal for a given species. Codon-optimized coding regions can be designed by various different methods known to one having ordinary skill in the art.

In certain embodiments, an entire polypeptide sequence, or fragment, variant, or derivative thereof is codon-optimized by any method known in the art. Various desired fragments, variants or derivatives are designed, and each is then codon-optimized individually. In addition, partially codon-optimized coding regions of the present invention can be designed and constructed. For example, the invention includes a nucleic acid fragment of a codon-optimized coding region encoding a polypeptide in which at least about 1%, 2%, 3%, 4%, 5%, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 100% of the codon positions have been codon-optimized for a given species. That is, they contain a codon that is preferentially used in the genes of a desired species, e.g., a yeast species such as S. cerevisiae, in place of a codon that is normally used in the native nucleic acid sequence.

In additional embodiments, a full-length polypeptide sequence is codon-optimized for a given species resulting in a codon-optimized coding region encoding the entire polypeptide, and then nucleic acid fragments of the codon-optimized coding region, which encode fragments, variants, and derivatives of the polypeptide are made from the original codon-optimized coding region. As would be well understood by those of ordinary skill in the art, if codons have been randomly assigned to the full-length coding region based on their frequency of use in a given species, nucleic acid fragments encoding fragments, variants, and derivatives would not necessarily be fully codon-optimized for the given species. However, such sequences are still much closer to the codon usage of the desired species than the native codon usage. The advantage of this approach is that synthesizing codon-optimized nucleic acid fragments encoding each fragment, variant, and derivative of a given polypeptide, although routine, would be time consuming and would result in significant expense.

In some embodiments, one or more of the donor parent polynucleotide sequences are codon-optimized for expression in yeast. In some embodiments, the chimeric polynucleotide is codon-optimized for expression in yeast.

V. Methods of Producing Ethanol

Certain aspects of the present invention are directed to methods of producing a fermentation product. In some embodiments of the invention, the recombinant host cell is used to produce a fermentation product from a cellulosic or lignocellulosic material. In some embodiments, the fermentation product is ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, a cephalosporin, or a combination thereof. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof.

One aspect of the invention is directed to a composition comprising a lignocellulosic material and a recombinant yeast host cell comprising at least one polypeptide having xylose isomerase activity and comprising a mutation in a gene encoding a protein associated with iron metabolism. Another aspect of the invention is directed to a media supernatant generated by incubating a recombinant yeast host comprising as least one polypeptide having xylose isomerase activity and comprising a mutation in a gene encoding a protein associated with iron metabolism with a medium containing xylose as the only carbon source. In some embodiments, the medium comprises a cellulosic or lignocellulosic material. In some embodiments, the cellulosic or lignocellulosic material is insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, saw mill or paper mill discards, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, corn fiber, agave, or a combination thereof.

In some embodiments, a fermentation product is produced by a method comprising contacting a recombinant host cell of the present invention with a carbon source, wherein the carbon source comprises xylose. In some embodiments, the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, and a cephalosporin. In some embodiments, the fermentation product is ethanol. In some embodiments, the fermentation product is recovered.

Certain aspects of the present invention are directed to a method of producing ethanol comprising contacting a source material comprising xylose with a host cell of the present invention. In some embodiments the host cell heterologously expresses a polypeptide having xylose isomerase activity. In some embodiments the host cell further comprises a mutation in one or more genes encoding a polypeptide that is associated with iron metabolism.

In some embodiments, the source material is a cellulosic biomass. In some embodiments, the source material is a lignocellulosic biomass. In some embodiments, the source material is selected from the group consisting of insoluble cellulose, crystalline cellulose, pretreated hardwood, softwood, paper sludge, newspaper, sweet sorghum, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, rice straw, nut shells, banana waste, sponge gourd fibers, corn fiber, agave, trees, corn stover, wheat straw, sugar cane bagasse, switchgrass, and combinations thereof. In some embodiments, the source material is corn stover.

EXAMPLES

The invention now being generally described, it will be more readily understood by reference to the following examples, which are included merely for purposes of illustration of certain aspect and embodiments of the present invention, and are not intended to limit the invention.

Example 1—S. cerevisiae Background Strain

A strain of S. cerevisiae was created that was suitable for the testing of functional xylose isomerases. The GRE3 locus of an industrial yeast strain was replaced with expression cassettes for the pentose phosphate pathway genes RPE1, RKI1, TKL1, and TAL1 as well as the native S. cerevisiae xyulokinase XKS1 (FIG. 1).

Example 2—Identification of Iron Metabolism Related Genes Mutated in Xylose Utilizing Strains

Specific mutations in three native S. cerevisiae genes (ISU1, YFH1, and NFS1) were identified that significantly improve performance of XI xylose engineered strains. The mutations were identified by reverse engineering several strains adapted for improved growth rate on xylose media. The adapted strains were derived from strains engineered to express an exogenous XI and to overexpress the native genes XKS, RKI1, RPE1, TAL1, and TKL1. Two strains were adapted that differed in the native GRE3+ locus, with one strain having a deletion of the endogenous GRE3. The mutations can be directly engineered into a strain providing the performance improvements usually obtained via adaptation. The directed engineering of these mutations saves the time and uncertainty associated with strain adaptations. These mutations can benefit strains engineered with various XIs (see FIGS. 5 and 6).

Example 3—Mutations in YFH1, ISU1, and NFS1 Improve Growth on Xylose

Strains were grown on YPX media (yeast extract, peptone, and xylose) under anaerobic conditions in a Biotek plate reader. OD600 measurements were used to determine changes in cell density over time (˜48 hrs) (FIG. 3). Xylose Utilizing Strains (XUS) 1 and 2 are strains engineered to utilize xylose but without mutations in YFH1, ISU1, or NFS1. XUS1-1 and XUS1-2 strains were adapted for improved growth on xylose originating from strain XUS1. Strain XUS2-1 was adapted for improved growth on xylose originating from strain XUS2. Genome sequencing revealed mutations in iron-sulfur cluster related genes in the adapted strains XUS1-1 (YFH1), XUS1-2 (NFS1) and XUS2-1 (ISU1). Direct genetic engineering to revert the mutations to the wild type alleles (XUS1-1->YFH1 wt, XUS2-1->ISU1 wt, XUS1-2->NFS1 wt) decreased xylose growth, matching the original parent strains. Direct genetic engineering of the iron-sulfur mutations into the parent strains (XUS1->YFH1 T163P, XUS2->ISU1 D71N, XUS1->NFS1 L115W) resulted in improved xylose growth matching the adapted strains with the same parent and mutation. The ISU1 D71N mutation was direct engineered as a heterozygote to match the mutation found in the adapted strain XUS2-1.

Example 4—Homozygousing the ISU1^(D71N) Mutation Improves Growth on Xylose

Strains were grown on YPX media (yeast extract, peptone, and xylose) under anaerobic conditions in a Biotek plate reader. OD600 measurements were used to determine changes in cell density over time (˜48 hrs) (FIG. 4A). The negative control is a strain that is unable to grow on xylose. Adapted strain XUS2-1 is heterozygous at the ISU1 locus. XUS2-1 genetically engineered with two mutant alleles of ISU1^(D71N) (XUS2-1+ISU1* homo) exhibits improved growth on xylose relative to the original heterozygote XUS2-1. Engineering the original parent strain with two mutant alleles of ISU1^(D71N) (XUS2+ISU1* homo) results in improved xylose growth equivalent to the XUS2-1ISU1^(D71N) homozygote.

Example 5—The Homozygous ISU1^(D71N) Mutation Improves Growth of the XUS1 GRE3⁺ Parent Strain

Strains were grown on YPX media (yeast extract, peptone, and xylose) under anaerobic conditions in a Biotek plate reader. OD600 measurements were used to determine changes in cell density over time (˜48 hrs) (FIG. 4B). The negative control is a strain that is unable to grow on xylose. The ISU1^(D71N) mutation was identified as a heterozygous mutation in an adapted xylose-utilizing strain with GRE3 deleted (XUS2-1). Direct engineering of the ISU1^(D71N) heterozygous mutation into the GRE3⁺ xylose strain XUS1 did not improve xylose growth (data not shown). Engineering XUS1 strain with two mutant alleles of ISU1^(D71N)(XUS1+ISU1* homo) results in significantly improved xylose growth equivalent to the XUS2 directly engineered ISU1^(D71N)homozygote (XUS2+ISU1* homo). Strain XUS1-1 is an adapted version of XUS1 containing a homozygous mutation in YFH1. XUS1-1 directly engineered homozygous ISU1^(D71N) exhibits decreased performance.

Example 6—The YFH1^(T163P) Mutation Improves Growth of the Yeast Strains Heterologously Expressing Various XIs

Strains were grown on YNBX minimal media, and the OD600 was measured following 48 hours of aerobic growth at 35° C. (FIG. 5). Various XIs were expressed on plasmids within the industrial host strain used for the chimeric XI library (black bars) or the host strain plus the YFH1 T163P Fe/Su cluster mutation (hashed bars). Eight colonies from each transformation were inoculated into YNBX media. Nearly all of the XIs that generated growth above the negative control, which lacked an XI, showed a benefit from the presence of the YFH1 mutant allele.

In a second set of experiments, strains were grown on YPX media (yeast extract, peptone, xylose) under anaerobic conditions in a Biotek plate reader at 35° C. OD600 measurements were used to determine changes in cell density over time (˜48 hours) (FIGS. 6 A and B). The negative control is a strain unable to grow on xylose. FIG. 6A shows strains containing the wild type allele of YFH1. FIG. 6B shows strains containing the YFH1T163P allele. All of the XIs tested using this genomic integration format showed significantly improved growth on xylose with the YFH1T163P allele present. CX355=chimeric xylose isomerase 355, CX1224=chimeric xylose isomerase 1224, Ad=Abiotrophia defectiva, Bt=Bacteroides thetaioatomicron, Pe=Piromyces, Ls=Lachnoanaerobaculum saburreum

Example 7—Mutations in AFT1 and CCC1 Improve Xylose Growth

Strains were grown on YPX media (yeast extract, peptone, and xylose) under anaerobic conditions in a Biotek plate reader. OD600 measurements were used to determine changes in cell density over time (˜48 hrs) (FIG. 7). The negative control is a strain that is unable to grow on xylose. Xylose utilizing strain (XUS) is a strain engineered to utilize xylose. XUS1-1 strain was adapted for improved growth on xylose originating from strain XUS1 and was found by genome sequencing to contain a mutation in iron-sulfur cluster related gene YFH1; XUS1-1 serves as a positive control. Direct engineering of the AFT1-1UP allele into the XUS1 strain (XUS1+AFT1-1UP) slightly improved growth on xylose. Direct engineering of the AFT1-1UP allele into and deletion of both endogenous copies of CCC1 in the XUS1 strain (XUS1+AFT1-1UP, ccc1Δ) result in significantly improved xylose growth close to that of the XUS1-1 strain.

Example 8—Addition of Iron Improves Growth on Xylose

Strains were grown on SP1 media (yeast nitrogen base with amino acids, tri-sodium citrate, glucose, xylose) under anaerobic conditions in serum bottles. Samples were taken and measured for ethanol, xylose and glucose concentrations over time (˜65 hours) (FIG. 8). Xylose Utilizing Strain 2 (XUS2) is engineered to utilize xylose. Strain XUS2-1 was adapted for improved growth on xylose originating from XUS2. Genome sequencing revealed mutations in iron-sulfur cluster related gene ISU1 in strain XUS2-1. Samples indicated as “+iron” were supplemented with iron at the start of the fermentation. The strains consumed all of the glucose at similar rates during the first ˜18 hours of the fermentation and produced similar amounts of ethanol with no difference seen with the addition of iron. In contrast, the addition of iron significantly improved the rate of xylose utilization as seen in the increased ethanol production between 18 and 65 hours. The increased xylose utilization (and subsequent ethanol production) was seen for both strains with and without the mutations in the iron-sulfur cluster related genes.

Example 9—Iron Addition Enables Significant Activity of Xylose Isomerase In Vitro

Xylose isomerase functions as a tetramer with the binding of two divalent cations per subunit essential for enzyme activity. Mg2+, Mn2+, Co2+, and Fe2+ ions activate the enzyme (Waltman et al. Protein Engineering, Design & Selection, 2014, p. 1-6). Using an in vitro enzymatic assay, the addition of Fe2+ was found to result in significantly more xylose isomerase activity than the addition of Mg2+ (FIG. 9). The protocol was essentially the same as described in Zou et al (Metabolic Engineering. 14, 2012, p. 611-622) with the exception of the use of three different buffers for the assay which varied in the absence or presence of the divalent metals Mg2+ or Fe2+. A cell extract was made from strain XUS1 which expresses the Bacteroides thetaiotaomicron xylose isomerase. The cell extract was combined with Tris buffer +/−divalent metals, NADH, and sorbitol dehydrogenase. The assay was initiated with the addition of xylose and the reaction was monitored for 2 minutes at 340 nm to determine the initial rate. The reactions were performed under inert atmosphere and reducing conditions to deter oxidation of Fe2+ to Fe3+. One unit of activity is equal to 1 umol NADH oxidized/min/ml, which corresponds directly with the consumption of the xylose that is added to initiate the reaction.

All documents cited herein, including journal articles or abstracts, published or corresponding U.S. or foreign patent applications, issued or foreign patents, or any other documents, are each entirely incorporated by reference herein, including all data, tables, figures, and text presented in the cited documents.

Following are particular embodiments of the disclosed invention

E1. A recombinant yeast cell comprising (a) at least one heterologous gene encoding a protein associated with iron metabolism and/or one or more mutations in one or more endogenous gene encoding a protein associated with iron metabolism; and (b) at least one heterologous gene encoding a polypeptide having xylose isomerase activity.

E2. The recombinant yeast cell of E1, wherein the at least one heterologous gene encoding a protein associated with iron metabolism and/or the one or more mutations in one or more endogenous gene encoding a protein associated with iron metabolism confers on the recombinant yeast cell an increased ability to utilize xylose as compared to a similar yeast cell lacking the one or more mutations.

E3. The recombinant yeast cell of E1 or E2, wherein the one or more mutations is a heterozygous mutation.

E4. The recombinant yeast cell of E1 or E2, wherein the one or more mutations is a homozygous mutation.

E5. The recombinant yeast cell of any one of E1-E4, wherein the recombinant yeast cell is a member of a genus selected from the group consisting of Saccharomyces, Kluyveromyces, Candida, Pichia, Schizosaccharomyces, Hansenula, Kloeckera, Schwanniomyces, and Yarrowia.

E6. The recombinant yeast cell of claim E5, wherein the recombinant yeast cell is a member of a species selected from the group consisting of Saccharomyces cerevisiae, Saccharomyces bulderi, Saccharomyces exiguus, Saccharomyces uvarum, Saccharomyces diastaticus, Candida krusei, Kloeckera lactis, Kloeckera marxianus, and Kloeckera fragilis.

E7. The recombinant yeast cell of claim E5, wherein the recombinant yeast cell is a member of a species selected from the group consisting of Saccharomyces cerevisiae, Saccharomyces bulderi, Saccharomyces exiguus, Saccharomyces uvarum, Saccharomyces diastaticus, Kloeckera lactis, Kloeckera marxianus, and Kloeckera fragilis.

E8. The recombinant yeast cell of any one of E1-E7, wherein the recombinant yeast cell is S. cerevisiae.

E9. The recombinant yeast cell of any one of E1-E4, wherein the one or more mutations in an endogenous gene is in a gene selected from the group consisting of ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, GREX4, CCC1, and any combination thereof.

E10. The recombinant yeast cell of E9, wherein the one or more mutations is a substitution of at least one nucleotide.

E11. The recombinant yeast cell of E10, wherein the recombinant yeast cell comprises one or more mutations in the endogenous ISU1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of D71N, D71G, and S98F, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:29.

E12. The recombinant yeast cell of E10 or E11, wherein the recombinant yeast cell comprises one or more mutations in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:31.

E13. The recombinant yeast cell of any one of E10-E12, wherein the recombinant yeast cell comprises one or more mutations in the endogenous NFS1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of L115W and E458D, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:33.

E14. The recombinant yeast cell of any one of E9-E13, wherein the recombinant yeast cell comprises a mutation in the endogenous AFT1 gene that results in increased Aft1 activity.

E15. The recombinant yeast cell of any one of E9-E14, wherein the recombinant yeast cell comprises a mutation in the endogenous AFT2 gene that results in increased Aft2 activity.

E16. The recombinant yeast cell of any one of E9-E15, wherein the recombinant yeast cell comprises one or more mutations in one or more endogenous genes selected from FRA1, FRA2, GREX3, and GREX4; wherein the one or more mutations results in increased activity of Aft1 and/or Aft2; and/or wherein the one or more mutations results in increased expression of one or more genes regulated by Aft1 and/or Aft2.

E17. The recombinant yeast cell of E16, wherein the recombinant yeast cell further comprises a mutation in an endogenous gene selected from the group consisting of YAP5 and CCC1.

E18. The recombinant yeast cell of E17, wherein the recombinant yeast cell comprises a deletion or disruption of YAP5 or CCC1.

E19. The recombinant yeast cell of any one of E1-E18, wherein the heterologous gene (a) is selected from the group consisting of AFT1, AFT2, and orthologues and combinations thereof.

E20. The recombinant yeast cell of any one of E1-E18, wherein heterologous gene (a) encodes a protein that increases the activity of Aft1 and/or Aft2 and/or increases the expression of AFT1 and/or AFT2.

E21. The recombinant yeast cell of E18, wherein the heterologous gene (a) encodes a protein that suppresses or inhibits the activity and/or expression of a protein that suppresses or inhibits the activity of Aft1 and/or Aft2 and/or suppresses or inhibits the expression of AFT1 and/or AFT2.

E22. The recombinant yeast cell of any one of E1-E18, wherein the heterologous gene (a) encodes a target of Aft1 and/or Aft2.

E23. The recombinant yeast cell of any one of E1-E18, wherein the heterologous gene (a) encodes a polypeptide having iron transport activity.

E24. The recombinant yeast cell of any one of E1-E23, wherein the heterologous gene (a) is constitutively expressed.

E25. The recombinant yeast cell of any one of E1-E24, wherein the heterologous gene (b) encodes a xylose isomerase enzyme.

E26. The recombinant yeast cell of E25, wherein the heterologous gene (b) encodes a polypeptide having at least 80% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

E27. The recombinant yeast cell of E25, wherein the heterologous gene (b) encodes a polypeptide having at least 80% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

E28. The recombinant yeast cell of E26, wherein the heterologous gene (b) encodes a polypeptide having at least 85% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

E29. The recombinant yeast cell of E27, wherein the heterologous gene (b) encodes a polypeptide having at least 85% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

E30. The recombinant yeast cell of E28, wherein the heterologous gene (b) encodes a polypeptide having at least 90% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

E31. The recombinant yeast cell of E29, wherein the heterologous gene (b) encodes a polypeptide having at least 90% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

E32. The recombinant yeast cell of E30, wherein the heterologous gene (b) encodes a polypeptide having at least 95% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

E33. The recombinant yeast cell of E31, wherein the heterologous gene (b) encodes a polypeptide having at least 95% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

E34. The recombinant yeast cell of E32, wherein the heterologous gene (b) encodes a polypeptide having 100% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 35, 37, 39, and 41.

E35. The recombinant yeast cell of E33, wherein the heterologous gene (b) encodes a polypeptide having 100% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, and 27.

E36. The recombinant yeast cell of any one of E1-E35, wherein the recombinant yeast cell further comprises at least one genetic modification of one or more endogenous genes encoding a protein of the pentose phosphate pathway.

E37. The recombinant yeast cell of E36, wherein the recombinant yeast cell comprises at least one genetic modification in at least one of the endogenous genes selected from the group consisting of XKS1, RKI1, RPE1, TKL1, and TAL1.

E38. The recombinant yeast cell of E37, wherein the recombinant yeast cell comprises one or more genetic modifications that leads to the overexpression of at least one of the endogenous genes selected from the group consisting of XKS1, RKI1, RPE1, TKL1, and TAL1.

E39. The recombinant yeast cell of any one of E1-E38, wherein the recombinant yeast cell further comprises a deletion or disruption of one or more aldose reductase genes.

E40. The recombinant yeast cell of E39, wherein the aldose reductase gene is GRE3 or YPR1.

E41. The recombinant yeast cell of E40, wherein the recombinant yeast cell comprises a deletion or disruption of GRE3 and YPR1.

E42. The recombinant yeast cell of any one of E1-E41, wherein the yeast cell further comprises a modification of the endogenous PGM1 gene.

E43. The recombinant yeast cell of E42, wherein the modification of the endogenous PGM1 gene results in the overexpression of PGM1.

E44. The recombinant yeast cell of any one of E1-E43, wherein the recombinant yeast cell is capable of growing on xylose as the sole carbon source.

E45. A method for producing a fermentation product comprising contacting the recombinant yeast cell of any one of E1-E44 with a carbon source, wherein said carbon source comprises xylose and/or xylan.

E46. A method for producing a fermentation product comprising contacting the recombinant yeast cell of any one of E1-E44 with a carbon source, wherein said carbon source comprises xylose.

E47. The method of E45, wherein the recombinant yeast cell is further grown on a media supplemented with iron.

E48. The method of E45 or E46, wherein the fermentation product is selected from the group consisting of ethanol, lactic acid, 3-hydroxy-propionic acid, hydrogen, butyric acid, acrylic acid, acetic acid, succinic acid, citric acid, malic acid, fumaric acid, an amino acid, 1,3-propane-diol, ethylene, glycerol, acetone, isopropyl alcohol, butanol, a β-lactam, an antibiotic, a cephalosporin, and combinations thereof.

E49. The method of E47, wherein the fermentation product is ethanol.

E50. The method of any one of E45-E48, further comprising recovering the fermentation product.

E51. A method of producing ethanol comprising contacting a carbon source comprising xylose and/or xylan with the recombinant yeast cell of any one of E1-E44 in a fermentation medium under conditions wherein ethanol is produced.

E52. A method of producing ethanol comprising contacting a carbon source comprising xylose with the recombinant yeast cell of any one of E1-E44 in a fermentation medium under conditions wherein ethanol is produced.

E53. The method of E50, wherein the fermentation medium is supplemented with iron.

E54. The method of E50 or E51, wherein the carbon source comprises cellulosic or lignocellulosic biomass.

E55. The method of E52, wherein the cellulosic or lignocellulosic biomass is selected from the group consisting of insoluble cellulose, crystalline cellulose, pretreated hardwood, paper sludge, pretreated corn stover, pretreated sugar cane bagasse, pretreated corn cobs, pretreated switchgrass, pretreated municipal solid waste, pretreated distiller's dried grains, pretreated wheat straw, corn fiber, agave, trees, corn stover, wheat straw, sugar cane bagasse, switchgrass, and combinations thereof.

E56. The method of E53, wherein the biomass is corn stover.

E57. The method of claim any one of E50-E54, further comprising recovering the ethanol.

E58. The recombinant yeast cell of any one of E1-E44 for use in a fermentation which convert a carbon source into a fermentation product, wherein said carbon source comprises xylose and/or xylan.

E59. The recombinant yeast cell of E35, wherein the recombinant yeast cell comprises heterologous expression of one or more polynucleotides encoding XKS1, RKI1, RPE1, TKL1, and/or TAL1.

Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims. 

1. A recombinant yeast cell comprising (a) at least one heterologous gene encoding a protein associated with iron metabolism and/or one or more mutations in one or more endogenous gene encoding a protein associated with iron metabolism; and (b) at least one heterologous gene encoding a polypeptide having xylose isomerase activity.
 2. The recombinant yeast cell of claim 1, wherein the one or more mutations in an endogenous gene is in a gene of ISU1, YFH1, NFS1, AFT1, AFT2, YAP5, FRA1, FRA2, GREX3, GREX4, CCC1, or any combination thereof.
 3. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell comprises one or more mutations in the endogenous ISU1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of D71N, D71G, and S98F, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:29.
 4. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell comprises one or more mutations in the endogenous YFH1 gene that results in a polypeptide comprising a T163P substitution, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:31.
 5. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell comprises one or more mutations in the endogenous NFS1 gene that results in a polypeptide comprising at least one amino acid substitution selected from the group consisting of L115W and E458D, wherein the position of the substitution is relative to the amino acid positions of SEQ ID NO:33.
 6. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell comprises a mutation in the endogenous AFT1 gene that results in increased Aft1 activity and/or a mutation in the endogenous AFT2 gene that results in increased Aft2 activity.
 7. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell comprises one or more mutations in one or more endogenous genes FRA1, FRA2, GREX3, or GREX4; wherein the one or more mutations results in increased activity of Aft1 and/or Aft2; and/or wherein the one or more mutations results in increased expression of one or more genes regulated by Aft1 and/or Aft2.
 8. The recombinant yeast cell of claim 2, wherein the recombinant yeast cell further comprises a mutation in an endogenous gene selected from the group consisting of YAP5 and CCC1.
 9. The recombinant yeast cell of claim 1, wherein the heterologous gene (a) is selected from the group consisting of AFT1, AFT2, and orthologues and combinations thereof.
 10. The recombinant yeast cell of claim 1, wherein heterologous gene (a) encodes a protein that increases the activity of Aft1 and/or Aft2 and/or increases the expression of AFT1 and/or AFT2 and/or suppresses or inhibits the activity and/or expression of a protein that suppresses or inhibits the activity of Aft1 and/or Aft2 and/or suppresses or inhibits the expression of AFT1 and/or AFT2.
 11. The recombinant yeast cell of claim 1, wherein the heterologous gene (a) encodes a target of Aft1 and/or Aft2.
 12. The recombinant yeast cell of claim 1, wherein the heterologous gene (a) encodes a polypeptide having iron transport activity.
 13. The recombinant yeast cell of claim 1, wherein the heterologous gene (b) encodes a polypeptide having at least 80%, 85%, 90%, 95% or 100% sequence identity with an amino acid sequence selected from the group consisting of SEQ ID NOs: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, and
 37. 14. The recombinant yeast cell of claim 1, wherein the recombinant yeast cell further comprises at least one genetic modification of one or more endogenous genes encoding a protein of the pentose phosphate pathway.
 15. The recombinant yeast cell of claim 14, wherein the recombinant yeast cell comprises at least one genetic modification in at least one of the endogenous genes selected from the group consisting of XKS1, RKI1, RPE1, TKL1, and TAL1.
 16. The recombinant yeast cell of claim 15, wherein the recombinant yeast cell further comprises a deletion or disruption of one or more aldose reductase genes.
 17. The recombinant yeast cell of claim 16, wherein the aldose reductase gene is GRE3 or YPR1.
 18. The recombinant yeast cell of claim 17, wherein the yeast cell further comprises a modification of the endogenous PGM1 gene.
 19. A method for producing a fermentation product comprising contacting the recombinant yeast cell of claim 1 with a carbon source, wherein said carbon source comprises xylose and/or xylan.
 20. The recombinant yeast cell of claim 1, wherein the recombinant yeast cell comprises heterologous expression of one or more polynucleotides encoding XKS1, RKI1, RPE1, TKL1, and/or TAL1. 